Satellite communication systems typically employ large aperture antennas and high power transmitters for establishing an uplink to the satellite. Recently, however, very small aperture antenna ground terminals, referred to as remote ground terminals, have been developed for data transmission at low rates. In such systems, the remote ground terminals are utilized for communicating via a satellite from a remote location to a central hub station. The central hub station communicates with multiple remote ground terminals, and has a significantly larger antenna, as well as a significantly larger power output capability than any of the remote ground terminals.
Typically, the remote ground terminals comprise a small aperture directional antenna for receiving and transmitting signals to a satellite, an outdoor unit ("ODU") mounted proximate the antenna for generating a modulated carrier signal to be transmitted to the central hub station and for receiving signals from the central hub station, and an indoor unit ("IDU") which demodulates the received signals and which also operates as an interface between a specific user's communication equipment and the outdoor unit.
Because the viability of the remote ground terminal concept increases as the cost for providing the remote ground terminal at the remote location decreases, it is necessary to decrease the cost of the remote ground terminal as much as possible. Furthermore, in order to decrease the operational costs, it is also necessary to decrease the size, weight and overall complexity of the remote ground terminal. Remote terminals with antenna diameters smaller than approximately 2 meters are often referred to as very small aperture terminals, or VSATs.
VSAT technology has followed a steady progression of cost and size reduction since the concept was first introduced in the early 1980s. Some of the earliest VSATs used a transmitter design topology known as "direct modulation" in which the RF carrier is modulated directly within the ODU. This concept provided several advantages over the previous earth station design practice in which a modulated IF carrier is fed to the ODU from the IDU and upconverted. Some drawbacks exist with direct modulation however, in that the VSAT's synthesizer (typically comprised of two parts: a "wideband loop" and a "narrowband loop") must be located in the outdoor equipment which is subject to more environmental extremes (temperature, vibration, etc.) than the indoor equipment. Also, due to the large number of circuits in the ODU, AC power is typically required at the outdoor equipment. A further disadvantage of this approach is that the modulating data (the transmitter's input signal) and the synthesizer control signals must be conveyed to the ODU via some means. This was typically done with an expensive multi-pair cable running between the IDU and ODU (the intra-facility link, or IFL), although other multiplexing schemes have been used.
Later VSATs reduced the size and component count in the ODU by relocating the synthesizer to the IDU. In this design, the ODU used a phase-locked-loop (PLL) to develop an RF local oscillator (LO) signal. This LO signal was used to upconvert the IF signal fed from the IDU. The advantages of a smaller ODU package are readily apparent, but a wideband (typically 500 MHz) IFL cable between the IDU and the ODU is required. This cable is costly and limits the maximum distance between the IDU and ODU. Also, control and stabilization of the output power level is more complex than in the previous example.
A third type of known VSAT design falls somewhere between the previous two in concept. In this third approach, the synthesizer is split with the narrowband loop in the IDU and the wideband loop in the ODU. The modulation is performed in the IDU to minimize the amount of equipment located outdoors. The cost of the IFL cable can be reduced because it is not necessary to carry a wide bandwidth transmit signal to the ODU. A lower cost "narrowband" IFL cable can be used. There are several drawbacks to this approach. Because the wideband synthesizer is located in the ODU, it must withstand increased temperature and vibration. Also, the synthesizer control signals must be conveyed to the ODU. Furthermore, because the modulated IF signal from the IDU is conveyed as an analog waveform, it is subject to noise and other impairments, especially level variations, as it traverses the IFL cable.
Accordingly, there exists a need to improve upon the preciously discussed techniques to provide a VSAT in which: 1) no synthesizers are required in the ODU; 2) a narrowband IFL cable can be used; 3) sensitivities to the noise and level variation distortions on the IFL are eliminated; and 4) the overall complexities and cost of the VSAT are minimized.